Photovoltaic cells based on nano or micro-scale structures

ABSTRACT

Novel structures of photovoltaic cells (also treated as solar cells) are provided. The cells are based on nanometer-scaled wires, tubes, and/or rods, which are made of electronic materials covering semiconductors, insulators or metallic in structure. These photovoltaic cells have large power generation capability per unit physical area over the conventional cells. These cells will have enormous applications in space, commercial, residential, and industrial applications.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/597,040 filed Nov. 6, 2005.

FIELD OF INVENTIONS

This patent specification relates to structures of photovoltaic cells(also solar cells). More specifically, it relates to structures ofphotovoltaic cells comprising numerous nanometer-scale wires, rodsand/or tubes to increase the junction area for increasing powergeneration capability per unit area. This also relates to photovoltaiccells comprising with nano or micro scaled-blocks. These photovoltaiccells can be used in commercial, residential, and also industrialapplications for power generation.

BACKGROUND OF THE INVENTIONS

Photovoltaic cells where light is converted into electric power and fedto external loads, which are electrically connected to the photovoltaiccells, have been prevailing in a wide range of applications such asconsumer electronics, industrial electronics, and space exploration. Inconsumer electronics, photovoltaic cells that consist of materials suchas amorphous silicon are used for a variety of inexpensive and low powerapplications. Typical conversion efficiency, i.e. the solar cellconversion efficiency, of amorphous silicon based photovoltaic cells isin the range of ˜10% [Yamamoto K, Yoshimi M, Suzuki T, Tawada Y, OkamotoT, Nakajima A. Thin film poly-Si solar cell on glass substratefabricated at low temperature. Presented at MRS Spring Meeting, SanFrancisco, April 1998.]. Although the fabrication processes of amorphoussilicon based photovoltaic cells are rather simple and inexpensive, onenotable downside of this type of cell is its vulnerability todefect-induced degradation that decreases its conversion efficiency.

In contrast, for more demanding applications such as residential andindustrial solar power generation systems, either poly-crystalline orsingle-crystalline silicon is typically used because there are morestringent requirements for better reliability and higher efficiency thanthe applications in consumer electronics. Photovoltaic cells consistingof poly-crystalline and single-crystalline silicon generally offerconversion efficiencies in the range of ˜20% and ˜25% [Zhao J, Wang A,Green M, Ferrazza F. Novel 19.8% efficient ‘honeycomb’ texturedmulticrystalline and 24.4% monocrystalline silicon solar cell. AppliedPhysics Letters 1998, 73: 1991-1993.] respectively. As many concernsassociated with a steep increase in the amount of the worldwide energyconsumption are raised, further development in industrial solar powergeneration systems has been recognized as a main focus for analternative energy source.

Group II-VI compound semiconductors, for example CdTe and CdS, have beenconsidered for the purpose of creating industrial solar power generationsystems, manufactured at a lower cost and moderate conversionefficiency. This approach resulted in a comparable conversion efficiencyof ˜17% [Wu X, Keane J C, Dhere R G, DeHart C, Duda A, Gessert T A,Asher S, Levi D H, Sheldon P. 16.5-efficient CdS/CdTe polycrystallinethin-film solar cell. Proceedings of the 17th European PhotovoltaicSolar Energy Conference, Munich, 22-26 Oct. 2001, 995-1000.]. Thisconversion efficiency is comparable to those for the single crystallinesilicon photovoltaic devises; however, the toxic nature of thesematerials is of great concern for environment.

Group I-III-VI compound semiconductors, such as CuInGaSe₂, have beenextensively investigated for industrial solar power generation systems.This material can potentially be synthesized at a much lower cost thanits counterpart, single crystalline silicon. However, a conversionefficiency of ˜19%, which is comparable to that of single crystallinesilicon based cells, can be obtained, thus far, only by combining withthe group II-VI compound semiconductor cells [Contreras M A, Egaas B,Ramanathan K, Hiltner J, Swartzlander A, Hasoon F, Noufi R. Progresstoward 20% efficiency in Cu(In,Ga)Se polycrystalline thin-film solarcell. Progress in Photovoltaics. Research and Applications 1999, 7:311-316.], which again raises issues associated with the toxic nature ofthese materials.

Photovoltaic cells designed for several exclusive applications, wherethe main focus is high conversion efficiency, generally consist of groupIII-V semiconductors, including GaInP and GaAs. In general, synthesisprocesses of single crystalline group III-V are very costly because ofsubstantial complications involved in epitaxial growth of group III-Vsingle crystalline compound semiconductors. Typical conversionefficiencies of group III-V compound semiconductor based photovoltaiccells, as these types of photovoltaic cells are intended to be, can beas high as 34% when combined with germanium substrates, another veryexpensive material [King R R, Fetzer C M, Colter P C, Edmondson K M, LawD C, Stavrides A P, Yoon H, Kinsey G S, Cotal H L, Ermer J H, Sherif RA, Karam N H. Lattice-matched and metamorphic GaInP/GaInAs/Geconcentrator solar cells. Proceedings of the World Conference onPhotovoltaic Energy Conversion (WCPEC-3), Osaka, May 2003, to bepublished.].

All photovoltaic cells in the prior art described above, regardless ofwhat materials the cell is made from, essentially fall into one specifictype of structure, described in FIG. 1A. Shown in FIG. 1A is aphotovoltaic cell comprising a thick p-type semiconductor layer 101 anda thin n-type semiconductor layer 102 formed on an electricallyconductive substrate 100. A pn-junction 103 is formed at the interfacebetween the p-type semiconductor layer 101 and the n-type semiconductorlayer 102. Incident light 104 entering the cell generate electron-holepairs after being absorbed by the p- and also n-type semiconductorlayers 101 and 102. The incident light 104 generates electrons 105 e andholes 105 h in the region near the pn-junction 103 and also electrons106 e and holes 106 h in the region far from the pn-junction 103. Thephotogenerated electrons 105 e and 106 e (and holes) (hereafterconsidering only electronics, i.e. minority carriers in p-typesemiconductors, although the same explanation is applicable for holes,minority carriers in n-type semiconductors) diffusing toward thepn-junction 103 and entering the pn-junction 103 contribute tophotovoltaic effect. The two key factors that substantially impact theconversion efficiency of this type of photovoltaic cell are photocarrier generation efficiency (PCGE) and photo carrier collectionefficiency (PCCE).

The PCGE is the percentage of photons entering a cell which contributeto the generation of photo carriers, which needs to be, ideally, 100%.On the other hand, the PCCE is the percentage of photogeneratedelectrons 105 e and 106 e that reach the pn-junction 103 and contributeto the generation of photocurrent. For a monochromatic light, a PCGE of˜100% can be achieved by simply making the p-type layer 101 thicker thanthe n-type material; however, electrons 106 e generated at the regionfar away from the pn-junction 103 cannot be collected efficiently due tomany adverse recombination processes that prevent photogeneratedcarriers from diffusing into the pn-junction 103. Thus, the basicstructure of current photovoltaic cells has its own limitation onincreasing the conversion efficiency.

FIG. 1B shows typical monochromatic light intensity behavior 108 insidethe semiconductor. As illustrated in FIG. 1B, the light intensitybehavior 108 inside bulk semiconductor is exponential. The lightintensity p at certain depth x can be expressed as p(x)=P_(o)exp(−αx),where P_(o) is the peak intensity at the surface and α is the absorptionco-efficient of the semiconductor in which light is entering. Carriers(not shown here) generated due to light flux 112 absorbed by thepn-junction 103 is only drifted by the junction field and can becollected efficiently, whereas, carriers 106 e and 106 h generated dueto absorption of light-flux 110 by semiconductor region 101 are diffusedin all directions. Only those carriers 105 e and 105 h which aregenerated closer (a distance equal to or less than the diffusion-lengthof the semiconductor) to pn-junction 103 can be collected. Thosecarriers 106 e and 106 h which are generated far away (distance longerthan the diffusion-length of the semiconductor) from pn-junction 103 arerecombined and lost. The light flux 112 is usually lost either byleaving or being absorbed by the substrate.

Both PCGE and PCCE are largely dependent on the material and structureof the photovoltaic cells. Today's photovoltaic cells are structured insuch a way that (a) wide ranges of the solar spectrum cannot be absorbeddue to material limitations, and (b) PCCE is low due to its inherentstructure. For example, the typical conversion efficiency of today'scrystal-Si based solar cell is ˜18%. Wavelengths of the solar spectrumspread from <0.1 μm to 3.5 μm, but Si can only absorb ˜0.4 μm to 0.9 μmof light. 50% of light belonging to the solar spectrum cannot beabsorbed by Si due to its inherent material properties. The remaining32% is lost due to (i) recombination of photogenerated carriers and (ii)loss of light, which is represented by 112 in FIG. 1B; these two factorsare structurally dependent. If we could reduce these two factors, ˜50%conversion efficiency could be achieved, even in a Si-based solar cell.If we could capture different wavelengths of light belonging to thesolar spectrum by utilizing different alloyed materials, we couldincrease the conversion efficiency ideally to 100%. Furthermore, if thesolar cell detection capability could be extended to theinfrared-spectrum, then the solar cell could produce electrical energynot only during the day (while sun is present), but also at night(hereafter defined by when the sun is not out). Additionally, today'ssolar cell material is not highly radiation-tolerant. Specifically inspace applications, photovoltaic cells should be highly radiationtolerant and have structure and material systems which can generatehigh-power per unit area.

For both commercial and space applications, therefore, it would bedesirable to have photovoltaic cell structures where both the PCGE andPCCE can be increased simultaneously by having a photo absorption regionwhich is thick enough to capture all the photons entering the cell and apn-junction which is located as close to the photo absorption region aspossible. It would be further desirable to have, while maintaining idealPCGE and PCCE, materials which have photo responses at differentspectrums in order to efficiently cover a wide spectrum of light thatenters a photovoltaic cell. It would be further desirable to have alarge junction area within a given volume of a photovoltaic cell so thatgenerated electric power that is proportional to the junction area canbe maximized. It would be further desirable to have solar cells whichcould generate electric power in both day and night.

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide structures ofphotovoltaic cells which have high power generation capability per unitarea, with respect to the conventional counterparts mentioned as theprior art.

Accordingly, it is an object of this invention to reduce therecombination of carriers and increase the absorption of light, whicheffectively increases the photogenerated carriers.

It is an object of this invention to provide solar cell structures basedon nano or micro scaled-blocks, such as rods or wires formed on thesupporting substrate or formed on the electronic materials which areformed on the base substrate. The pn- or Schottky junctions are formedwith nano or micro scaled-blocks, which generate built-in potential bywhich photogenerated electrons and holes are swept away, leading tophotovoltaic effect.

According to this invention, the supporting substrate can be Si, GaAs,InP, GaN, glass, Ge, C, ZnO, BN, Al₂O₃, AlN, Si:Ge, CuInSe, II-VI andIII-V.

It is an object of this invention to have electronic materials on whichnano or micro scaled-blocks (rods, wires, or tubes) can be formed andthe electronic materials can be formed on a base substrate such as Si,Ge or glass, to decrease the cost.

According to this invention, it is also an object to use the nano ormicrometer scaled blocks to increase the surface area and also toincrease the amplifying or concentrating the light incident onto thesurface. The side of the nano or micrometer(s) scaled blocks could beany shape such as vertical or inclined at specific angle with respect tosubstrate surface.

It is an object of this invention to provide structures of photovoltaiccells which can capture most of the wavelengths belonging to the solarspectrum and can provide >80% conversion efficiency.

It is an object of this invention to provide structures of photovoltaiccells which can generate electric power when the sun is and is not out.

This invention offers to ideally achieve >50% conversion efficiencyutilizing Si-materials and >80% conversion efficiency for othermaterials. The main advantage of these inventions are that today'shighly matured semiconductor process technologies allow fabrication ofthe proposed photovoltaic cell which has much larger power generationcapabilities as compared to that of conventional photovoltaic cells.

Other objects, features, and advantages of the present invention will beapparent from the accompanying drawings and from the following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail in conjunction with theappended drawings wherein:

FIG. 1 shows the cross-sectional view of a conventional photovoltaiccell structure. This is the explanatory diagram representing today'sphotovoltaic cell and the light intensity behavior inside semiconductormaterials.

FIG. 2A is the schematic of the cross-sectional view of a photovoltaiccell structure consisting of the nanometer(s) or micrometer(s)-scaledrods, vertically arranged, in the first embodiment, in accordance withthe present invention. FIGS. 2B and 2C are the schematics representingthe principle solar cell as shown in FIG. 2A.

FIG. 3 is the schematic showing the cross-sectional view of aphotovoltaic cell structure consisting of the nanometer(s) ormicrometer(s)-scale rods, vertically arranged, in the second embodimentin accordance with the present invention.

FIGS. 4A and 4B are the schematics showing the cross-sectional views ofphotovoltaic cell structures consisting of the nanometer(s) ormicrometer(s)-scaled rods and semiconductors, bandgaps of which arerelated to different spectrums of the solar spectrum, in the thirdembodiment in accordance with the present invention.

FIGS. 5A and 5B are the schematics showing the cross-sectional views ofphotovoltaic cell structures consisting of the nanometer(s) ormicrometer(s)-scaled rods, formed on the standard low-cost substrate, inthe forth embodiment in accordance with the present invention.

FIGS. 6A, 6B, 6C, 6D, 6E, and 6F are the schematics showing thefabrication process flow of photovoltaic cells comprising of thenanometer(s) or micrometer(s)-scaled rods, in the fifth embodiment inaccordance with the present invention.

FIGS. 7A, 7B, 7C, 7D, and 7E are the schematics showing an alternativefabrication process flow of photovoltaic cells comprising of thenanometer(s) or micrometer(s)-scaled rods, in the sixth embodiment inaccordance with the present invention.

FIGS. 8A, 8B, 8C, 8D, 8E, and 8F are the schematics showing thefabrication process flow of flexible photovoltaic cells comprising ofthe nanometer(s) or micrometer(s)-scaled rods, in the seventh embodimentin accordance with the present invention.

FIGS. 9A and 9B are the schematics representing the enlargedcross-sectional view of the day and night solar cell in according to theinvention and panel of said solar cells, respectively.

DETAILED DESCRIPTION

According to the preferred embodiment illustrated in FIG. 2A, aphotovoltaic cell comprising of a plurality of nanometer(s) ormicrometer(s)-scaled rods 201 is formed on the supporting substrate 200(after having groove). The nanometer(s) or micrometer(s)-scaled rods 201can have metallic electrical conduction, p-type or n-type semiconductorelectrical conduction. The nanometer(s) or micrometer(s)-scaled rods 201are further surrounded by an electronic material 202 having metallicelectrical conduction, p-type or n-type semiconductor electricalconduction. The electronic material 202 can be a separate material or anelectronic material of p or n type formed inside 201 and 200. Theelectronic material 202 and the supporting substrate 200 areelectrically connected to electrodes 203 a and 203 b, respectively. Theelectrode 203 a is intended to serve as a common electrode that connectsall rods 201. The electrode 203 a is provided for the electronicmaterial 202. The electrode 203 a can be transparent (not shown here)and can be formed on the electronic material 202. The interface betweenthe nanometer scaled rods 201 and the electronic material 202 form pn-or Schottky junctions where built-in potential for both electrons andholes is generated.

Alternatively, according to this invention, the nanometer(s)-scaled rods201 can be formed on a separate substrate (not shown here), and theelectrode 203 b can be formed on the substrate to have common contactfor each nanometer(s)-scaled rod 201, necessary for creating a junction.

In way of an example not way of limitation, the nanometer(s)-scaled rods201 can be made of n-type semiconductor and the electric material 202that surrounds the nanometer(s)-scaled rods 201 can be made of p-typesemiconductor. Incident light 204 enters the photovoltaic cell througheither the electrode 203 a or on the electronic material 202. (In FIG.2, the incident light enters the photovoltaic cell through theelectronic material 202 and not the electrode 203 a). The incident light204 travels through the nano-scaled rods 201, electronic material (n- orp-type) 202, and substrate 200. As the incident light 204 travelsthrough the nano-scaled rods 201 and electronic material 202, numerouselectrons 205 e are generated in the region near the electrode 203 a.Portions of light 204 which pass through the gaps 207 travel through theelectronic material 202 and the supporting substrate 200, whichconsequentially generates electrons 206 e. Some electrons 206 e aregenerated closer to electronic material 202 and are collected, whilesome of which generate in the region far from the electronic material202 and are recombined and lost. It should be pointed out that electronsare apparently generated all over the region along the thickness of theelectric material 202. In addition, as the incident light 204 travelsthrough the nanometer(s)-scaled rods 201, numerous holes 205 h and 206 hare generated in the rods 201 and in the substrate 200, respectively. Italso should be pointed out that holes are apparently generated all overthe region along the thickness of the nanometer(s)-scaled rods 201 andthe substrate 200. Photogenerated electrons 205 e and 206 e in theelectronic material 202, rods 201, and substrate 200 diffuse towardpn-junctions, created at the interface between the nanometer(s)-scaledrods 201 and the electronic material 202, and also at the interfacebetween the electronic material 202 and substrate 200. At thepn-junctions, the electrons and the holes are swept away by built-inpotential, thus photovoltaic effects set in.

Unlike the conventional solar cell shown in FIG. 1A, the invention haspn-junctions in the regions on top 208 of the rods 201, on the side 210of the rods 201, and in the gaps 212 in between the rods 201, asdepicted in FIG. 2B. The pn-junction formed on the side 210 of the rods201 having a penetration depth of the incident light of H (FIG. 2C), anda variation of the light intensity along the penetration depth 214dependent on the penetration depth of the incident light H of the rods201. The light 204 travels parallel to the direction of the pn-junctionformed across side 210 of the rods and most of the light intensity alongthe penetration depth 214 incident on the pn-junction is absorbed andmost of the photogenerated carriers can be collected. The light 204travels perpendicular to the pn-junctions formed at the top 208 of therods 201 and in the gaps 212 in between the rods 201. Most of the lightflux incident on the top of the rods 208 can also be absorbed. Thecarriers generated by the light 204 incident on the top 208 of the rodscan be collected without recombination (ideally). Not all carriersgenerated by the light 204 incident on the gaps 212 in between the rods201 can be collected. It is apparent that utilizing the solar cell asshown in FIGS. 2A-2C can (i) reduce the recombination and (ii) absorball photo flux, thereby increasing the conversion efficiency.

The apparent advantage of this invention over conventional photovoltaiccells is directly associated with the fact that, unlike conventionalphotovoltaic cells, large portions of the pn-junctions are almostparallel to the direction to which incident light 204 travels. As such,the distance all photogenerated carriers in the electronic material 202,regardless of where they are generated, have to diffuse in order toreach the pn-junctions is within the range of the distance between twonanometer(s)-scaled rods existing next to each other and that distanceis independent of the location where they are generated. Furthermore,the distance all photogenerated carriers in the rods 201, regardless ofwhere they are generated, have to diffuse to reach pn-junctions iswithin the range of the diameter of the nanometer(s)-scale rods 201.Only in the gaps 212 formed in between the rods 201 does light 204travel perpendicular to the pn-junctions and, therefore, not all thecarriers can be collected. According to this invention, therecombination can be made zero (ideally) and all photon flux can beabsorbed (ideally), and the conversion efficiency can be made >50%, evenusing Si. Conventionally, as explained in the description of the priorart shown in FIG. 1, photovoltaic cells have pn-junctions that areperpendicular to the direction to which incident light travels.Therefore, the photogenerated carriers generated in the region far awayfrom pn-junctions need to diffuse a much longer distance(diffusion-length) than photogenerated carriers generated near thepn-junctions, thus having a greater chance to recombine withoutcontributing to photovoltaic effects. Therefore in this invention, PCCEis expected to be much higher than in conventional photovoltaic cells.In addition, it is evident that the total effective area whichcontributes to photovoltaic effects in this invention can be increasedsignificantly by a few orders (>3000), considering 300 mm diametersubstrate, 500 μm height rods having 50 nm diameter and 100 nm pitch.

According to this invention, in way of an example not way of limitation,the supporting substrate 200 can be n-type Si, on which n-type Si,Si—Ge, or Si-based alloyed rods 201 are formed after making the grooveson the front surface of the substrate 200. The p-type dopants can bediffused into the rods 201 and the substrate 200 regions can be openedto form the electronic material 202 of Si p-type. The metal contacts 203a and 203 b can be formed on the p-type Si 202. Conformal deposition ofthe dielectric material (not shown) can be done for planarization, andin this case silicon oxide or a polymer can be used.

According to this invention, in way of an example not way of limitation,the supporting substrate 200 can be Ge, GaAs, InP, GaN, or ZnO on whichrods 201 of same type of material can be formed.

In an alternative preferred embodiment shown in FIG. 3, a photovoltaiccell comprises a plurality of nanometer(s)-scaled rods 301 which areelectrically connected to a substrate 300. The nanometer(s)-scaled rods301 can have metallic electrical conduction, p-type or n-typesemiconductor electrical conduction. The nanometer(s)-scale rods 301 aresurrounded by an electronic material 302 having metallic electricalconduction, p-type or n-type semiconductor electrical conduction. Theelectronic material 302 can be separate material or an electronicmaterial of p or n type formed inside or on 301 and 300. The electronicmaterial 302 and the supporting substrate 300 are electrically connectedto electrodes 303 a and 303 b, respectively. The electrode 303 a isintended to serve as a common electrode that connects all rods 301. Theelectrode 303 a is provided for the electronic material 302. Theinterface between the nanometer(s)-scale rods 301 and the electronicmaterial 302 form pn- or Schottky junctions, thus there are pn- orSchottky junctions on both sides, inside and outside, of thenanometer(s)-scale rods 301.

Alternatively, according to this invention, the nanometer(s)-scale rods301 can be formed on a substrate (not shown here), and the electrode 303a can be made on the substrate to have a common contact for eachnanometer(s)-scale rod 301, necessary for creating a junction.

In way of an example not way of limitation, the nanometer(s)-scale rods301 can be made of metal and the electronic material 302 that surroundsthe nanometer(s)-scale rods 301 can be made of p-type semiconductor,thus the interface of 302/301 forms pn-junctions. Incident light 304enters the photovoltaic cell through the electronic material 302(front-side of the cell). As the incident light 304 travels through theelectronic material 302, numerous electrons 305 and 306 e (ofelectron-hole pairs) are generated. It should be pointed out thatelectrons (of electron-hole pairs) are apparently generated all over theregion along the thickness of the nanometer(s)-scale rods 301 and alsothe gaps 312 in between the rods 301. Photogenerated electrons in theelectronic material 302 then diffuse toward pn-junctions in theinterface of 302/301. At the pn-junctions, the diffused electrons areswept away by built-in potential, thus photovoltaic effects set in.

Common advantages already described for the photovoltaic cell in FIG. 2,can be similarity achieved in this embodiment. The only difference is informing the nano-scaled rods 301, which are formed without forming thegrooves.

In an alternative preferred embodiment shown in FIG. 4A, a photovoltaiccell comprises a plurality of nanometer(s)-scaled rods 401 which areelectrically connected to a substrate 400. The nanometer(s)-scaled rods401 can have metallic electrical conduction, p-type or n-typesemiconductor electrical conduction. The nanometer(s)-scale rods 401 aresurrounded by an electronic material 402 having metallic electricalconduction, p-type or n-type semiconductor electrical conduction. Theelectronic material 402 can be separate material or an electronicmaterial of p or n type formed on 401 and 400. The electronic material402 and the supporting substrate 400 are electrically connected toelectrodes (not shown here). The interface between thenanometer(s)-scale rods 401 and the electronic material 402 formpn-junctions 408, thus creating built-in-potential for collectingphotogenerated carriers.

In way of an example not way of limitation, the nanometer(s)-scale rods401 can be made of metal and the electronic material 402 that surroundthe nanometer(s)-scale rods 401 can be made of p-type semiconductor,thus the interface of 402/401 forms pn-junctions 408. Incident light 404enters the photovoltaic cell through the electronic material 402(front-side of the cell). As the incident light 404 travels through theelectronic material 402, numerous electrons 405 and 406 (ofelectron-hole pairs) are generated in the electronic material 402. Itshould be pointed out that electrons 405 and 406 (of electron-holepairs) are apparently generated all over the region along the thicknessof the electronic material 402. Photogenerated electrons in theelectronic material 402 then diffuse toward the pn-junctions in theinterface of 402/401. At the pn-junctions, the diffused electrons areswept away by built-in potential, thus photovoltaic effects set in.

According to this invention, in way of an example not way of limitation,the supporting substrate 400 can be n-type InP, on which n-type InP orInP-based alloyed rods 401 are formed on the front surface of thesubstrate 400. The p-type InGaAs layer(s) having broad spectralabsorption, ranging from as low as <0.3 μm to as high as 2.5 μm, is usedfor the electronic material 402. The metal contacts (not shown here) canbe formed on InGaAs and the substrate 400. Conformal deposition of thedielectric material (not shown) can be done for planarization, and inthis case silicon oxide or a polymer can be used. Use of single ormultiple layers of InGaAs helps absorb more wavelengths of light, from<0.3 μm to 2.5 μm, which range belongs to the solar spectrum.

According to this invention, in way of an example not way of limitation,the supporting substrate 400 can be GaAs, GaN, or ZnO on which rods 401of the same type of material can be formed.

In an alternative preferred embodiment, shown in FIG. 4B, a photovoltaiccell comprises a plurality of nanometer(s)-scaled rods 401 which areelectrically connected to a supporting substrate 400. Thenanometer(s)-scaled rods 401 can have metallic electrical conduction,p-type or n-type semiconductor electrical conduction. Thenanometer(s)-scale rods 401 are further surrounded by multi-layerelectronic materials 402 a and 402 b (two or more), having absorption indifferent spectrum bands and also having metallic electrical conduction,p-type or n-type semiconductor electrical conduction. The electronicmaterials 402 a and 402 b can be separate material or an electronicmaterials of p or n type formed on 401 and 400. The electronic materials402 a and 402 b and the supporting substrate 400 are electricallyconnected to electrodes (not shown here). The interfaces between thenanometer(s)-scale rods 401/402 a and between 401/402 b formpn-junctions 408, thus creating built-in-potential for collectingphotogenerated carriers. Apparently, in addition to the commonadvantages already described in FIG. 2-FIG. 3 over conventional cells inFIG. 1, the additional advantage of the cell in FIG. 4B is a capabilityof covering a wide range of the spectrum contained in incident light 404and converting a wide range of the spectrum to photogenerated carriers.Dozens of different layers could be stacked to catch photons at allenergies, to make absorption wide enough to cover the entire solarspectrum, from lower wavelengths (as low as X-ray) to longer wavelength(e.g. long infrared). The addition of multiple junctions of differentmaterials, which could absorb a wide solar spectrum, plus the increasein the junction area caused by using the rods 401, will help to increasethe conversion efficiency close to 100% (ideally). According to thisinvention, dozens of materials, which could absorb wide ranges of thesolar spectrum, may or may not require a lattice mismatch with the rod,wires, or tubes. Lattice matched material could further increase thepower generation due to reduction of the recombination.

According to this invention, in way of an example not way of limitation,the supporting substrate 400 can be n-type InP, on which n-type InP orInP-based alloyed rods 401 are formed on the front surface of thesubstrate 400. The p-type InGaAs layer(s) having broad spectralabsorption, ranging from as low as <0.3 μm to as high as 2.5 μm, andInSb based electronic material, are used as electronic materials 402 aand 402 b of p-type, respectively. The metal contacts (not shown here)can be formed on InGaAs and substrate 400. Conformal deposition of thedielectric material (not shown) can be done for planarization, and inthis case silicon oxide or a polymer can be used. Use of the single ormultiple layers of the InGaAs, and other antimony based electronicmaterials, help absorb more wavelengths of light, from <0.3 μm to 3.5μm, which range belongs to the solar spectrum.

According to this invention, in way of an example not way of limitation,the supporting substrate 400 can be GaAs, GaN, or ZnO on which rods 401of same type of material can be formed.

In an alternative preferred embodiment shown in FIG. 5A, a photovoltaiccell comprises a plurality of nanometer(s)-scaled rods 501, which areelectrically connected to a substrate 500. The nanometer(s)-scaled rods501 can have metallic electrical conduction, p-type or n-typesemiconductor electrical conduction. The nanometer(s)-scale rods 501 aresurrounded by an electronic material 502 having metallic electricalconduction, p-type or n-type semiconductor electrical conduction. Theelectronic material 502 can be a separate material or an electronicmaterial of p or n type formed on 501 and 500. The electronic material502 and the supporting substrate 500 are further electrically connectedto electrodes (not shown here). The interface between thenanometer(s)-scale rods 501 and the electronic materials 502 formpn-junctions 508, thus creating built-in-potential for collectingphoto-generated carriers. The main difference between the solar cellsshown in FIGS. 4A and 5A, is that electronic material 520 is formed onthe low cost supporting substrate 500 (e.g. Si or AlN).

In way of an example not way of limitation, the nanometer(s)-scale rods501 can be made of metal and the electronic material 502 that surroundsthe nanometer(s)-scale rods 501 can be made of p-type semiconductor,thus the interface of 502/501 forms pn-junctions 508. Incident light 504enters the photovoltaic cell through the electronic material 502(front-side of the cell). As the incident light 504 travels through theelectronic material 502, numerous electrons 505 and 506 (ofelectron-hole pairs) are generated in electronic material 502. It shouldbe pointed out that electrons 505 and 506 (of electron-hole pairs) areapparently generated all over the region along the thickness of theelectronic material 502. Photogenerated electrons in the electronicmaterial 502 are made of p-type semiconductor, and then diffuse towardpn-junctions 508 in the interface of 502/501. At the pn-junctions, thediffused electrons are swept away by built-in potential, thusphotovoltaic effects set in.

According to this invention, in way of an example not way of limitation,the supporting substrate 500 can be n-type Si, on which lattice matchedInP or InP based alloys are formed. Next, n-type InP, or InP-based alloyrods 501 are formed. The p-type InGaAs layer(s), having broad spectralabsorption which ranges from as low as <0.3 μm to as high as 2.5 μm, isformed as an electronic material 502 of p-type. The metal contacts (notshown here) can be formed on InGaAs and substrate 500. Conformaldeposition of the dielectric material (not shown) can be done forplanarization, and in this case silicon oxide or polymer can be used.Using single or multiple layers of the InGaAs helps absorb morewavelengths of light, from <0.3 μm to 2.5 μm, of which belong to thesolar spectrum.

According to this invention, in way of an example not way of limitation,the supporting substrate 500 can be Ge, GaAs, GaN, or ZnO.

In an alternative preferred embodiment shown in FIG. 5B, a photovoltaiccell comprises a plurality of nanometer(s)-scaled rods 501, which areelectrically connected to a supporting substrate 500. Thenanometer(s)-scaled rods 501 can have metallic electrical conduction,p-type or n-type semiconductor electrical conduction. Thenanometer(s)-scale rods 501 are surrounded by an electronic material502, having metallic electrical conduction, p-type or n-typesemiconductor electrical conduction. The electronic material 502 can bea separate material or an electronic material of p or n type formed on501 and 500. The electronic material 502 and the supporting substrate500 are further electrically connected to electrodes (not shown here).The interface between the nanometer(s)-scale rods 501 and the electronicmaterial 502 form pn-junctions 508, thus creating built-in-potential forcollecting photogenerated carriers. The main difference between thesolar cells a shown in FIGS. 4B and 5B, is that electronic material 520is formed on the low cost supporting substrate 500 (e.g. Si or AlN).

According to this invention, in way of an example not way of limitation,the supporting substrate 500 can be n-type Si, on which lattice matchedInP or InP based alloys are formed. Next, n-type InP, or InP-based alloyrods 501 are formed. The p-type InGaAs layer(s), having broad spectralabsorption which ranges from as low as <0.3 μm to as high as 2.5 μm, andInSb based electronic material are formed as electronic materials 502 aand 502 b of p-type, respectively. The metal contacts (not shown here)can be formed on InGaAs and substrate 500. Conformal deposition of thedielectric material (not shown) can be done for planarization, and inthis case silicon oxide or polymer can be used. Using single or multiplelayers of the InGaAs and other antimony based electronic material, helpsabsorb more wavelengths of light, from <0.3 μm to 3.5 μm, of whichbelong to the solar spectrum.

According to this invention, in way of an example not way of limitation,the supporting substrate 500 can be Ge, GaAs, GaN, or ZnO.

FIGS. 6A, 6B, 6C, 6D, and 6E are the schematics showing the fabricationprocess of the photovoltaic cell according to this invention, whereinthe same numerals in FIGS. 6A-E represent the same parts in FIG. 2, sothat similar explanations are omitted. According to this invention,supporting substrate 200 can be crystal-Si, Ge, GaAs, or InP. Afterstandard photolithography and wet-etching, grooves 210 are formed ontothe front surface of the substrate 200. A thin metal layer (not shownhere) is formed, which is followed by the patterning and etching to formseveral nano-scaled lines inside the grooves 210. High temperatureannealing can be used to form nano-sized metals 212 which can be used asa catalyst for forming the rods 201. Using conventional chemical vapordeposition techniques, the rods 201 are formed. It is noted here thatusing grooves 210 helps control the size distribution and density of therods 201. Using a diffusion process, dopants (e.g. p-type) are diffusedto form the pn-junction. For making the planarization, conformaldeposition of the silicon-oxide or polymer can be used (not shown here).The final stages make the planarization, using insulator layer 220, andcontacts 203 a and 203 b. Both contacts 203 a and 203 b can be takenfrom the back side of the substrate 200 after planarization forcompleting the solar cell 215, as shown in FIG. 6F.

FIGS. 7A, 7B, 7C, 7D, and 7E are the schematics showing the fabricationprocess of the solar cell according to this invention. Here, as in FIGS.6A-F, the metal lines 711 patterns are created for making thenano-metals 712. The only difference with what was explained in FIGS.6A-F, is that no grooves are made on the substrate 700. The explanationof the fabrication process and the structure of the cells is previouslyexplained in FIGS. 2-6, so that repeated explanation is omitted here.

FIGS. 8A, 8B, 8C, 8D, 8E, and 8F are the schematics showing thefabrication process of the flexible solar cell according to thisinvention. Here, as in FIGS. 6A-F and FIGS. 7A-E, the metal lines 811patterns are created for making the nano-metals 812. The only differencewith what was explained in FIGS. 7A-E, is that substrate 800 is etchedout after coating with conducting polymer. The explanation hereafter isalready explained in the discussion of FIGS. 7A-E, so that repeatedexplanation is omitted here.

FIGS. 9A and 9B are the schematics representing the enlargedcross-sectional view and large panel of the day and night photovoltaiccell, respectively and in accordance with the invention. The day andnight solar cell can be a single or hybrid cell discretely integrated.In the single cell case (not shown here), the cell can be fabricated tocapture a wide range of the spectrum, not only the solar spectrum butalso the thermal spectrum, which is especially necessary when the sun isnot out. In the hybrid case, as shown in FIG. 9A, the solar cell 915 aand the solar cell 915 b are designed for separate day and nightpurposes and they are discretely integrated using the common panel 930,which helps to connect other solar cells together. The solar cell 915 acan be designed (as mentioned above in the preferred embodiments) insuch a way that most of the solar spectrum is captured. For example, inthe case of c-Si, the cell can capture all visible and near infrared(IR) spectrums of light. On the other hand, the solar cell 915 b can bedesigned (as mentioned above in the preferred embodiments) in such a waythat most of the infrared spectrum and thermal spectrum can be captured.HgCdTe based material can be used for capturing a wide IR spectrum.HgCdTe on Si can also be used for obtaining a low cost objective. InGaAsbased material can also be used as an absorption material for capturingvisible to near-infrared spectrums.

A series of solar cells 915 a and 915 b can be connected on each side ofthe panel as shown in FIG. 9B, in such as way that the commonconnections 934 a and 934 b for solar cell 915 a are connected in seriesor in parallel with others, and similarly for connections 932 a and 932b for a series of solar cells 915 b. The fabrication process andstructure of solar cells 915 a and 915 b are previously explained in thediscussion of FIGS. 2-6, so that repeated explanation is omitted here.

According to this invention, the rods, or wires, could be GaN materials(n or p type) and the dozens of materials could be In_(1-x)Ga_(x)N (p orn type, opposite to GaN rods). With increasing of the Ga content, theband-gap of InGaN can be increased to 3.4 eV, which is the same as thatof the GaN. By increasing the In content in InGaN, the band gap can bereduced to ˜0.65 eV. Photons with less energy than the band gap willslip right through. For example, red light photons are not absorbed byhigh-band-gap semiconductors. However, photons with energy higher thanthe band gap are absorbed—for example, blue light photons in alow-band-gap semiconductor will be absorbed—their excess energy iswasted as heat.

According to this invention, alternatively the rods, or wires, could beIII-V based materials (n or p type). For example InP and the dozens ofmaterials could be III-V based material, for example In_(1-x)Ga_(x)As (por n type, opposite to InP rods). In this case, by adjusting the Incontents, the band gap can be tuned and thereby a wide spectrum of thesolar energy can be absorbed.

According to this invention, alternatively the rods, or wires, could beII-V based materials (n or p type). For example CdTe and the dozens ofmaterials could be II-VI based material, for example CdZnS (p or n type,opposite to CdTe rods). In this case, by adjusting the Zn contents, theband gap can be tuned and thereby a wide spectrum of the solar energycan be absorbed.

According to this invention, alternatively the rods, or wires, could beSi or amorphous Silicon materials (n or p type), and the dozens ofmaterials could be Si: Ge alloys (p or n type, opposite to Si rods). Inthis case, by adjusting the Ge contents, the band gap can be tuned andthereby a wide spectrum of the solar energy can be absorbed.

According to this invention, alternatively the rods, or wires, could beSi, InP, or CdTe (n or p type), and the dozens of materials could bedifferent material which could create the junction with the rods (wiresor tubes). Each type of material has a specific band gap for absorbingthe specific range of the solar spectrum. In this way a wide range ofthe solar spectrum can be absorbed and by increasing the junction area(due to use of the rods, wires, or tubes), the electrical powergeneration could be increased tremendously: 50 times and beyond.

According to this invention, the nanometer(s)-scale wires, rods ortubes, mentioned in the preferred embodiments, can be any kind ofelectronic materials including semiconductors, insulators or metals.

According to this invention, the nanometer sized rods, wires, or tubes,can be made from semiconductors such as Si, Ge, or compoundsemiconductors from III-V or II-VI groups. As an example for rods, wire,or tubes, a InP, GaAs, or GaN III-V compound semiconductor can be usedand they can be made using standard growth processes, for example,MOCVD, MBE, or standard epitaxial growth. According to this invention,the self-assembled process can also be used to make wires, rods, ortubes and their related pn-junction to increase the junction area. Theserods, wires, or tubes can be grown on the semiconductors (under samegroup or others), polymers, or insulators. Alternatively, according tothis invention, these rods, wires, or tubes, can be transferred to theforeign substrate or to the layer of foreign material. The foreignsubstrate or the layer of material can be any semiconductor such as Si,Ge, InP, GaAs, GaN, ZnS, CdTe, CdS, ZnCdTe, HgCdTe, etc. The substratecan also cover all kinds of polymers or ceramics such as AlN,Silicon-oxide, etc.

According to this invention, the nanometer sized rods, wires or tubes,based on II-VI compound semiconductors can also be used. As an exampleCdTe, CdS, Cdse, ZnS, or ZnSe can be used, and they can be made usingstandard growth processes, for example, sputtering, evaporation, MOCVD,MBE, or standard epitaxial growth. According to this invention, theself-assembled process can also be used to make wires, rods, or tubesand their related pn-junctions to increase the junction area. Theserods, wires, or tubes can be grown on the semiconductors (under samegroup or others), polymers, or insulators. Alternatively, according tothis invention, these rods, wires, or tubes can be transferred to theforeign substrate or to the layer of foreign material. The foreignsubstrate or the layer of material can be any semiconductor such as Si,Ge, InP, GaAs, GaN, ZnS, CdTe, CdS, ZnCdTe, HgCdTe, etc. The substratecan also cover all kinds of polymers or ceramics such as AlN,Silicon-oxide, etc.

According to this invention, the rods, wire, or tubes, mentioned earlierto make the photovoltaic cell, can be micro or nano scaled and theirsides could be vertical or inclined (in shape) at an angle (e.g α) withrespect to the surface of substrate. Alternatively, the side could benay shape convenient to manufacturing and increase the surface area. Theadvantage of using the inclined side is to concentrate the incidentlight falling onto the side and the gap in between the rods, wires, ortubes.

According to this invention, the nanometer sized rods, wires, or tubescan be made from carbon type materials (semiconductors, insulators, ormetal like performances) such as carbon nano-tubes, which could besingle or multiple layered. They can be made using standard growthprocesses, for example, MOCVD, MBE, or standard epitaxial growth.According to this invention, the self-assembled process can also be usedto make wires, rods, or tubes and their related pn-junctions to increasethe junction area. These tubes can be grown on the semiconductors (undersame group or others), polymers, or insulators. Alternatively, accordingto this invention, these rods, wires, or tubes, can be transferred tothe foreign substrate or to the layer of foreign material. The foreignsubstrate or the layer of material can be any semiconductor such as Si,Ge, InP, GaAs, GaN, ZnS, CdTe, CdS, ZnCdTe, HgCdTe, etc. The substratecan cover also all kinds of polymers or ceramics such as AlN,Silicon-oxide, etc.

Whereas many alterations and modifications of the present invention willno doubt become apparent to a person of ordinary skill in the art afterhaving read the foregoing description, it is to be understood that theparticular embodiments shown and described by way of illustration are inno way intended to be considered limiting. Therefore, reference to thedetails of the preferred embodiments is not intended to limit theirscope.

Although the invention has been described with respect to specificembodiments for complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching here isset forth.

The present invention is expected to of practical use as novelphoto-voltaic cells with higher power generation capability (25 timesand beyond), as compared to the conventional cells. The proposedinvention can be used for fabricating wide solar panels for bothcommercial and space applications.

1. A photovoltaic cell comprising: at least one substrate; a periodic ornon-periodic lattice of at least one electrically conductive material,e1, of some shape, which is formed on said substrate; at least oneelectrically conductive material, e2, which forms a pn-junction withsaid e1; and at least one transparent electrically conductive materialformed on said e2.
 2. A photovoltaic cell according to claim 1 whereinsaid substrate is an n-type semiconductor.
 3. A photovoltaic cellaccording to claim 1 wherein said electrically conductive material, e1,is a n or p-type semiconductor.
 4. A photovoltaic cell according toclaim 1 wherein said electrically conductive material, e2, is an p orn-type semiconductor.
 5. A photovoltaic cell according to claim 1wherein said lattice comprises nano or micro-sized rods made of saidelectrically conductive material, e1.
 6. A photovoltaic cell accordingto claim 5 wherein said nano or micro-sized rods form a pn-junction withsaid electrically conductive material, e2.
 7. A photovoltaic cellaccording to claim 6 wherein said nano-sized rods are surrounded by saidelectrically conductive material, e2.
 8. A photovoltaic cell accordingto claim 1 wherein at least one electrode is attached to saidphotovoltaic cell.
 9. A photovoltaic cell according to claim 1 whereinsaid e1 and said e2 are the same material, but one is doped n-type andthe other is doped p-type.
 10. A photovoltaic cell according to claim 1wherein said lattice is made of said e1 which is shaped in some way thatis convenient for manufacturing and/or in some way that attempts tomaximize the surface area.
 11. A series of photovoltaic cells accordingto claim 1 wherein said photovoltaic cells are designed using differentmaterials that will capture different wavelengths of light, therebycovering most of the infrared, thermal, and/or solar spectrums and saidphotovoltaic cells are electrically connected.
 12. A series ofphotovoltaic cells according to claim 11 wherein said photovoltaic cellsimplement InGaAs, HgCdTe, and/or Si as said electrically conductivematerial, e1, to capture respective spectrums of light.
 13. Aphotovoltaic cell comprising: at least one substrate which is etched tocreate grooves of some shape; at least one electrically conductivematerial, e1, which forms a pn-junction with said substrate; and atleast one transparent electrically conductive material formed on saide1, wherein said grooves are vertical or inclined at specific angle withrespect to the surface of said substrate.
 14. A photovoltaic cellaccording to claim 13 wherein said substrate is an n-type semiconductor.15. A photovoltaic cell according to claim 13 wherein said e1 is ap-type semiconductor.
 16. A photovoltaic cell according to claim 13wherein at least one electrode is attached to said photovoltaic cell.17. A photovoltaic cell according to claim 13 wherein said grooves areshaped in some way that is convenient for manufacturing and/or in someway that attempts to maximize the surface area.
 18. A series ofphotovoltaic cells according to claim 13 wherein said photovoltaic cellshave more than one junctions and they are designed using differentmaterials that will capture different wavelengths of light, therebycovering most of the infrared, thermal, and/or solar spectrums and saidphotovoltaic cells are electrically connected.
 19. A series ofphotovoltaic cells according to claim 18 wherein said photovoltaic cellsimplement InGaAs, HgCdTe, and/or Si as said substrate material tocapture respective spectrums of light.
 20. A method of manufacturing aphotovoltaic cell comprising: thin layers of at least one metal or metalalloyed layer is formed on a substrate; high temperature treatment toform the particles from the said metal or metal alloyed layer; saidparticles are used as catalyst to grow into rods by some depositionprocedure; a diffusion process is used to dope said rods, or anelectrically conductive material, e1, of opposite doping from said rodsis deposited on said rods to form pn-junctions; and a transparent layerof some electrically conductive material is formed over saidelectrically conductive material, e1.
 21. A method of manufacturing aphotovoltaic cell according to claim 20 wherein said depositionprocedure is chemical vapor deposition.
 22. A method of manufacturing aphotovoltaic cell according to claim 20 wherein at least one conductivecontact is attached to said photovoltaic cell.
 23. A method ofmanufacturing a photovoltaic cell according to claim 20 whereinconformal deposition is used to smooth said transparent layer.
 24. Amethod of manufacturing a photovoltaic cell comprising: a dopedsubstrate is etched to form grooves; an electrically conductivematerial, e1, of opposite doping from said substrate is deposited onsaid substrate and said grooves; a transparent layer of someelectrically conductive material, e2, is formed over said electricallyconductive material, e1, wherein said grooves are vertical or inclinedat specific angle with respect to the surface of said substrate.
 25. Amethod of manufacturing a photovoltaic cell according to claim 21wherein at least one conductive contact is attached to said photovoltaiccell.